spapr: Rename DT functions to newer naming convention

spapr: Rename DT functions to newer naming convention

In the spapr code we've been gradually moving towards a convention that
functions which create pieces of the device tree are called spapr_dt_*().
This patch speeds that along by renaming most of the things that don't yet
match that so that they do.

For now we leave the *_dt_populate() functions which are actual methods
used in the DRCClass::dt_populate method.

While we're there we remove a few comments that don't really say anything
useful.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Cédric Le Goater <clg@kaod.org>